This invention relates to magnetic resonance (MR) techniques. More specifically, this invention relates, in the preferred embodiment, to a calibration method for MR magnetic field gradient eddy current compensation filters. The invention is applicable to magnetic resonance imaging, but is not limited thereto.
The magnetic resonance phenomenon has been utilized in the past in high resolution magnetic resonance spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, MR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as in performing in vivo, noninvasive spectroscopic analysis. As is now well known, the MR phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, B.sub.o, by irradiating the object with radio frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or different RF coil is used to detect the MR signals, frequently in the form of spin echoes, emanating from the patient volume lying within the field of the RF coil. In the course of a complete MR scan, a plurality of MR signals are typically observed. The signals are used to derive MR imaging or spectroscopic information about the object studied.
The application of magnetic resonance to imaging depends upon the use of magnetic field gradients to encode spatial information within the NMR signal. Any departure from the ideal gradient behavior can be expected to introduce image distortion. For example, imperfect rephasing of the nuclear spins and an attendant loss of signal occurs if the gradients are not constant during selective time reversal pulse sequences (i.e., use of 180.degree. time-reversal RF pulses). This effect compounds in later spin echoes of multi-echo (Carr-Purcell-Meiboom-Gill) sequences. In addition, if the gradient field is not zero when it should be (due, e.g., to residual decay from a previous gradient pulse) the unitended phase dispersion can result in distorted spectra in chemical shift imaging (CSI) sequences as well as inaccurate spin-spin relaxation time (T.sub.2) determination in multi-echo sequences.
Such gradient distortions can arise if the gradient fields couple to lossy structures within the magnet, its cryostat (if the magnet is of superconductive design) or shim coil system, or the RF shield used to decople the gradient coils from the RF coil. The spurious response components derive from induction of currents in the ambient structures and/or loss of energy to the shim coils and are manifested as multi-compartment relaxation behavior. One observes, for example, an approximately exponential rise and decay of gradient fields during and after, respectively, the application of a rectangular current pulse to the gradient coil.
A scheme has been proposed which, in one embodiment, uses an analog pre-emphasis filter in the gradient power supply to shape the current applied to the gradient coil in such a way that the spurious gradient components are reduced. The filter may have a number (e.g., two) exponential decay components and adjustable potentiometers (e.g., four) which must be set during system calibration. A measurement technique is used to sample the uncorrected residual gradient field and calculate the potentiometer settings from analysis of the data. Other embodiments are envisioned wherein the current pulse may be shaped by techniques other than pre-emphasis-type filters, such as by generation of the desired current pulse shape by a computer.
Accordingly, it is an object of the present invention to provide a method for measuring the uncorrected field and to use this information to compensate for the gradient distortion.
It is another object of the invention to provide an improved method which uses NMR to measure the gradient response and which offers improved sensitivity.
It is a further object of the invention to provide an NMR method to measure the gradient response by monitoring the free-induction decay (FID) signal which reflects the entire integrated gradient history.